Conventionally, a synthetic resin coating with high corrosion resistance is applied on an inner surface of a metal container for protecting the inner surface of the metal container from its contents. As an example of such a metal container with an inside coating film, a metal can 100 with an inside coating film shown in FIG. 15 is known. This metal can 100 comprises a can 101 and a coating film 102 provided on the inner surface. The can 101 is plastic-deformed body of an aluminum sheet by stamping comprising a cylindrical body (side wall) 103, a bottom 104 closing the lower end, a truncated cone like shoulder 105 continuing from the upper part of the cylindrical body, and a cylindrical neck part 106 extending upward from the shoulder. A carting (a bead) 107 winding outside is provided at a top of the neck part 106. A thread 106a is formed around the neck part.
The manufacturing method of the metal can 100 is, in the case of a drawing-ironing can, a blank material produced by circularly punching out a thin metal sheet is processed by deep drawing to form a bottomed cylinder; and the bottomed cylinder is processed by ironing to make its body thin. Further the inner surface of the obtained bottomed cylindrical can is coated with the coating film 102 by powder coating or spray coating. After the coating film is formed, the shoulder 105 and the neck part 106 are formed by the necking processing. And then, the curling 107 is formed by the curling processing. In the case of a drawing-ironing can having a thread part, the shoulder and the thread part are formed by the spinning processing.
In the case of an impact can having a thread part used for an aerosol container, a thick metal sheet is punched out circularly, its shape is case by case arranged to form a slug, and an impact processing is applied to form a bottomed cylinder. And then, an inner surface is coated, the shoulder and the neck are formed by necking processing, and a bead part is formed by the curling processing. These processes are same as those of drawing-ironing can.
As described above, the inside coating films are formed after the plastic forming process of the metal sheets and slugs into the shape of a bottomed cylinder. Further after the coating, the shoulder and the neck part are formed by the necking processing (post processing). The reason why the inside coating film is formed before the post processing is because, when the inner surface is coated after the post processing, it may cause surface irregularity or partial lack of coatings resulting in unsatisfactory corrosion-resistance performance of the inside coating films, of which purpose is to protect the metal container from corrosion due to the contents.
As for the inside coating films 102, various coating films materials may be used depending on the property and purpose of the contents. For example, in the case that coating films having corrosion-resistibility of the metal, and excellent workability/adhesiveness to the can body is desired, phenolic epoxy resin may be coated. In this case, it is proposed that the drawing processing is provided to a pre-coated material in which the bottom of the can is coated with oil-based resin type coating film containing zinc oxide powder, and that the body is separately coated with phenolic epoxy resin type coating films (Refer to Japanese Unexamined Patent Publication No. 1986-47344). This invention is intended for prevention of inside sulfide foxing of the metal container due to marine products. In the Japanese Unexamined Patent Publication No. 1993-138125, a powder coating process is disclosed, in which the bottom and the can body are separately coated and baked at one time. And further, in this process, the same coating material can be coated separately and baked at one time.
In the Japanese Patent No. 2562343, a two layers inside coating technology for an aerosol product made of steel filled with a water-based aerosol composition is disclosed, wherein the priming layer is composed of 1 to 8 μm thick phenolic epoxy resin type coating compounds and the over coating layer is composed of 3 to 20 μm thick vinyl organosol resin type coating compounds whose glass transition temperature is higher than 50° C. These technologies are said to be able to prevent occurrence of cracks in the necking processing or in the double seam processing of the bottom and body. Further, the inside-coated can having primary layer with thickness of 1 μm or less composed of water based phenolic epoxy resin type coating compound and surface layer with thickness of 2 to 20 μm composed of water based/water dispersible phenolic epoxy acrylic resin type coating compound is disclosed in Japanese Unexamined Patent Publication H03-72578.
Further, the other inside-coated metal can manufacturing technologies are known, wherein a pre-coated material having two layers of coating film is processed by ironing (DRD processing, DI processing). For example, a food container for foods containing sulfur (Japanese Unexamined Patent Publication No. 1981-32237) is known, wherein phenolic epoxy resin type coating compound containing AI pigment is coated as the priming layer and the surface of the priming layer is covered with polyolefin films containing zinc oxide powder or unsaturated polyester films. And further, a metal can restraining the absorption of the pigment contained in foods such as crabs is known (Japanese Unexamined Patent Publication No. 1990-242743), wherein vinyl organosol resin type coating compound is coated as the priming layer and phenolic epoxy resin type coating compound is coated as the surface layer; and baked two times.
In the two times coating, a technology to prevent drooping of crude hardened coating films or occurrence of pinholes/cracks is known, wherein preparative drying or interim drying is carried out between the first and the second coating (Japanese Patent Publication No. 1988-44026, Japanese Patent Publication No. 1989-43584, Japanese Unexamined Patent Publication No. 2000-104013 etc.)
On the other hand, technology using UV ink is known and used for coated plywood boards and the likes (Japanese Unexamined Patent Publication No. 1973-60175. Japanese Unexamined Patent No. 1986-290099). However, since the UV ink contains photo polymerization initiator and photosensitizer, it is not suitable for inside coating of food containers or containers due to unknown safety to human body and due to stability problems in storage. In the Japanese Unexamined Patent Publication No. 1997-173898, a metal food container is disclosed, in which an ultraviolet ray-hardened type resin is printed on the outer surface of the container; a polymeric monomer not containing photo polymerization initiator and photosensitizer is over coated; and ultraviolet rays are illuminated to harden the coating, thus avoiding the photo polymerization initiator and photosensitizer to be contained on the outer surface of the container. However, the UV ink is not used for the inside coating of containers.
In collapsible tubes of metal type, a manufacturing technology is disclosed, in which forming the priming layer on the inner surface of the tube with epoxy resin type coating compound, and forming the covering layer on the priming layer by powder coating, so as to form thin film in the part of clinching (the coating film as the priming layer is allowed to expose)(Japanese Unexamined Utility Model Publication No. 1984-16850). This is directed to save the powder coating compound and to improve the visual appearance. Further, in order to improve the sealing performance of the clinching part, a tube container coated by powder coating is known, in which the clinching part of the inner tube is left uncoated (Japanese Unexamined Patent Publication No. 1980-116558).
The conventional metal containers, to which the corrosion-resistive inside coating is applied after the forming of cylindrical shape and then severe deformation processing or plastic processing such as necking, curling, or threading is applied, may cause the occurrence of crack or peelings of the inside coating films in the thread part or the necking part. Even in the case that no crack or peelings is found by visual check, the bad result of the electric flow test (high current flow in the current flow test) may be obtained due to the microscopic cracks or pinholes.
This invention is directed to the improvement of processing resistibility of corrosion-resistant inside-coated metal containers in the post processing, in which to reduce microscopic cracks and peelings as far as possible, and to obtain better electric current flow test results. Further, this invention is directed to provide a manufacturing method of metal containers having good durability to the post processing and having high corrosion resistivity to its contents.